The overarching goal of the proposed studies is to establish standards for a nutritionally healthy, captive common marmoset (Callithrix jacchus). The proposed research is significant because the potential of a valuable nonhuman primate model in the study of human diseases - the common marmoset - is being hindered by an incomplete understanding of the nutritional requirements of the species. The studies will identify critical features of a standardized basic diet for captive common marmosets; determine links between diet, gut microbiome, and disease; and establish standards for healthy weights, body condition, and biomarkers of metabolic function. These goals will be accomplished through the following three specific aims to be conducted in collaboration between the two national primate research centers (NPRCs) that maintain marmosets (Southwest and Wisconsin). The studies will take advantage of the natural experiment offered by the transfer of the nutritionally distinct marmoset population presently at New England NPRC to each of the Southwest and Wisconsin NPRCs: The project has three specific aims: 1: Conducting nutritional assessments regarding select macro- and micro-nutrient requirements for marmosets and their relations to biomarkers of gut function. These studies will establish normative biomarkers, explore the effects of broad dietary alterations on biomarkers and the gut microbiome, determine the effects of gluten removal on subjects with initial low digestive efficiency and determine vitamin D requirements through depletion-repletion studies; 2: Validating markers of healthy weight and metabolic function in captive common marmosets. These studies will determine the effects of body weight and sampling method on fasting glucose values, compare biomarkers of insulin resistance with an aim toward identifying readily accessible biomarkers and determine relations among body mass, fat mass, morphometrics and cage-side body scoring and functions encoded in the gut microbiome. This aim will determine the most appropriate weight range for adult common marmosets and answer the question of whether obesity is associated with particular functions encoded in the gut microbiome. 3: Identifying links between nutrition and disease in captive common marmosets, concentrating on the most common chronic disease states - inflammatory bowel diseases and inflammatory kidney diseases - as well as more novel diseases that may be important biomedical models - hepatic steatosis, diabetes and atherosclerosis. By taking advantage of established networks of investigators as well as research centers maintaining marmoset colonies, we will have sufficient population size to potentially establish links between dietary parameters and disease occurrence or, at a minimum, identify trends that will guide a subsequent, more targeted and expansive search for nutritional causes of disease.